Effective underwater adhesives can bring many benefits to industries or activities which deal with underwater operations. Applications such as attaching sensors to surfaces which are located in a body of water, repairing wet tissues, or patching leaky underwater oil pipelines can be improved. However, establishing underwater adhesion up to this point has proven to be problematic. In the field of adhesion science, water or moisture has traditionally been treated as surface contaminants or weak boundary layers. Synthetic adhesives perform poorly on wet surfaces or when used underwater due to a variety of complex mechanisms of deterioration including, but not limited to, erosion, plasticization, swelling, and hydrolysis of the adhesive polymers. Even though some may have strong bulk cohesive strength underwater, these adhesives tend to eventually fail due to the wicking and crazing of water due to poor interfacial adhesions.
Many of the solutions focusing on establishing underwater adhesion have been inspired by discoveries and phenomena found in nature. For instance, in the sea, there is a diversity of organisms that specialize in sticking to all type of wet surfaces: mussels hang on with a handful of threads constructed to alleviate the mechanical mismatch between hard rock and their soft invertebrate body, barnacles glue calcareous base plates to rocks and boat bottoms, and sandcastle worms live in tubes composed of sand, shell fragments, and blobs of underwater proteinaceous glue.
The objective of using underwater adhesives for wet applications, i.e. using Barnacle cements for dental cements, has been pursued since at least the late 1960s. During the last decade, new strategies for fabricating multi-functional underwater adhesives were developed by exploiting the adhesive characteristics of catechol functional groups.
There are two general approaches to the fabrication of catechol functionalized adhesives. In the first approach, adhesives are made by conjugating catechol functional groups to polymers such as disclosed in, at least, US Patent Publication No. 2003/0087338 entitled Adhesive DOPA-containing Polymers and Related Methods of Use, US Patent Publication No. 2005/0288398 entitled Polymeric Compositions and Related Methods of Use, U.S. Pat. No. 8,227,628 entitled Method of Synthesizing Acetonide-Protected Catechol-Containing Compounds and Intermediates Produced Therein and U.S. Pat. No. 7,943,703 entitled Modified Acrylic Block Copolymers for Hydrogels and Pressure Sensitive Wet Adhesives. Other references relating to this topic include U.S. Pat. No. 6,506,577 entitled Synthesis and Crosslinking of Catechol Containing Copolypeptides, U.S. Pat. No. 7,622,533 entitled Biomimetic Compounds and Synthetic Methods Therefor and US Patent Publication No. 2009/0036611 entitled Cross-Linkable Polymeric Compositions.
The second approach is a bottom-up process where adhesives are produced by the expression and purification of recombinant mussel adhesive proteins such as disclosed in U.S. Pat. No. 7,622,550 entitled Mussel Bioadhesive.
The applications of catechol functionalized adhesives can be divided into three categories. In the first category, the adhesives are used to replace commercial medical sealants such as disclosed in US Patent Publication No. 2008/0247984 entitled DOPA-Functionalized, Branched, Poly(Aklylene Oxide). Adhesives; US Patent Publication No. 2011/0027250 entitled Sealants for Fetal Membrane Repair, US Patent Publication No. 2010/0137902 entitled Bioadhesive Constructs; U.S. Pat. No. 5,665,477 entitled Hydrogel, Adhesive for Attaching Medical Device to Patient and U.S. Pat. No. 7,943,703. More specifically, US Patent Publication No. 2008/0247984 and US Patent Publication No. 2011/0027250 describe branched catechol terminated polymers for tissue repair, US Patent Publication 2010/0137902 describes a catechol functionalized wrap for applications such as bone repair, and U.S. Pat. No. 5,665,477 describes catechol functionalized hydrogel for attaching medical devices to wet tissues.
In the second category, the adhesives are used as anti-fouling, anti-bacterial, and anchoring surface coatings such as disclosed in US Patent Publication No. 2008/0171012 entitled Fouling Resistant Coatings and Methods of Making Same, U.S. Pat. No. 8,293,867 entitled Substrate-Independent Layer-by-Layer Assembly Using Catechol-Functionalized Polymers, US Patent Publication No. 2010/0028718 entitled Surface-Immobilized Antimicrobial Peptoids and US Patent Publication No. 2011-0052788 entitled Antifouling Hydrogels, Coatings, and Methods of Synthesis and Use thereof.
In the third category, the adhesives are used as underwater glues. Polystyrene, which is not ordinarily a component of adhesives, was used to mimic mussel adhesive proteins by incorporating catechol side chains, for which the polymer displayed enhanced adhesion upon cross-linking such as disclosed in US Patent Publication No. 2009/0036611 entitled Cross-Linkable Polymeric Compositions. In the same manner, the adhesive proteins of the sandcastle worm was also mimicked by synthesizing polyelectrolyte analogs with the same side chain chemistries and molar ratios of catechol, amine, and phosphate; forming a complex coacervate that qualitatively mimicked the entire range of natural glue behaviors including underwater delivery, interfacial adhesion, and triggered solidification such as disclosed in U.S. Pat. No. 8,283,384 entitled Adhesive Complex Coacervates and Methods of Making and Using Thereof and US Patent Publication No. 2009/0036611.
All of the prior solutions mentioned above require either complicated chemical modifications or synthesis of long-chain polymers with catechol functional groups or require the user making these adhesives to have a specific set of skills. The sophisticated fabrication process makes transferring of these solutions into practical applications and especially integration of them with existing systems difficult. For instance, adding adhesive properties to an existing drug delivery vehicle (hydrogel) by chemical modification is not really an option as the modification can result in a complete change in the vehicles' physical and chemical properties, rendering the previous understanding of the system obsolete. In terms of adhesion, the solutions mentioned above, although they have successfully mimicked certain aspects of natural underwater glue in terms of fluid properties, solidification, and adhesion, none of them have displayed any practical performance, i.e. achieving significant bond strength within a reasonable curing time (<2 hours). For example, although the synthetic polyelectrolyte glues of sandcastle worm have displayed a shear bond strength several times the estimated bond strength of the natural adhesive, the adhesive was applied on wet, smooth, and acid-treated aluminum substrates in air, and then submerged in water with a curing time of 24 hours.
Therefore, a need exists for new synthetic adhesive materials that overcome one or more of these current disadvantages.